Hi, Ron. Insulation works by making dead air space--spaces where air is trapped and can't circulate to move cold air to warm places. Packing insulation actually makes it easier for cold to move. Insulation has to be light and fluffy for it to work. Since your walls are full now, you will not gain anything by packing more insulation in. Look how much work and money I just saved you.
However, there are 2 places you might be able to make the insulation more efficient. One is the spaces around doors, windows, electrical boxes on outside walls, sill cock pipes going to the outside thru basement walls and the like. If cold is moving thru these spaces, you could be loosing effeciency. Take the molding strips off around one window and see if there is any space like between the rough opening and the window frame that has not been filled with some kind of insulation. If there are, you can either put in small strips of unfaced pink batts or use the spray in foam. If one window looks pretty good, it is probably a good bet that the others are too.
Around electrical boxes and pipes, use the spray in foam and be sure to be very careful with this when it is in the liquid form. Follow manufacturers directions and use the suggested safety equipment. When you are done with it, put wet pipe cleaners in the nozzle and spray tube and you might be able to use more out of the can later--but I only manage this about every other time.
The biggest single potential area for heat loss is your attic. If your house design is such that you have a big open space up there, you can probably add more insulation easily. The code specifies R35 and I don't think you gain much by going too far over this. The same applies for electrical boxes and such as for the walls, though. Be sure you don't use a faced insulation up there. You want any humidity in your house to move up and out. There are all sorts of variations where you have living space near the roof.
Many gas and electric companies will do an energy audit on your house for little or no cost. If you still have concerns, have one done. Henry in MI